Polymers and copolymers of conjugated dienes such as polybutadiene, polyisoprene, and styrene-butadiene rubbers possess physical properties which make them suitable for many important applications such as synthetic rubbers and as additives to other polymeric systems such as, for example, high impact polystyrene (HIPS). HIPS can be manufactured by polymerizing styrene in the presence of 5-10% dissolved polybutadiene or butadiene copolymer. During the polymerization of the styrene, the polybutadiene or butadiene copolymer is grafted onto the styrene polymer chain. To increase the grafting efficiency of the polybutadiene, it is desirable for the polybutadiene to have end segments having a high vinyl content rather than cis or trans configurations, The vinyl content refers to alkenyl groups configured pendant to the polymer backbone, as opposed to cis and trans configurations which contain the alkenyl groups within the polymer backbone. The terms vinyl content and vinyl bond content are used interchangeably herein.
Polymers with high vinyl end segments can be produced by initiating polymerization and allowing the reaction to proceed to near completion. As the polymerization reaction approaches completion, additional monomer and a vinyl modifier are added. The vinyl modifer increases the number of vinyl configured bonds that are formed during polymerization, hence leading to increased vinyl content. The final segment of the polymer thus has a higher vinyl bond content than the beginning segment. The vinyl concentration of the end segment can be controlled to levels as high as about 70%. However, if one wishes both ends of the polymer, or the ends of a star branched polymer, to have high vinyl content with the remaining segments having low vinyl content, this method will not work because coupling reactions result in high vinyl in the center of the polymer chain or star branched polymer.
To produce linear or branched polymers with all ends having a high vinyl content, an organo-dilithium initiator can be used to polymerize a low vinyl middle segment. Subsequently, additional monomer and a vinyl modifier are added to produce ends having a high vinyl content. This method avoids the need for a coupling agent. However, organo-dilithium initiators are expensive and unstable.
Alternatively, linear or branched polymers with all ends having a high vinyl content may be produced by first forming a living prepolymer high vinyl initiator by anionic polymerization of monomer units in the presence of a vinyl modifier. The living prepolymer high vinyl initiator is then utilized to initiate polymerization of additional monomer units, thus producing a polymer having a high vinyl end and a low vinyl end. The low vinyl end is formed as a result of the extra monomer units lowering the concentration of the vinyl modifier. The polymers are then coupled between the low vinyl ends, producing a polymer with each end containing a high vinyl segment. However, if this method is used in a batch polymerization process, the transition from high vinyl to low vinyl is gradual since the change results from the decrease in concentration of the vinyl modifier upon addition of additional monomer. Since the transition is gradual, the higher the desired vinyl content in the end segment is, the higher the vinyl content in the middle portion of the polymer chain will be. Further, dilution of the modifier is not an effective method when chelating modifiers are used.
The above method may also be used in a continuous process, wherein the living prepolymer high vinyl initiator is made continuously in one reactor and fed to a second reactor where the bulk of the polymer is made. In comparison to a batch process, a continuous process utilizing the above method can produce a more abrupt change in vinyl content. However, the abrupt change is caused by using a higher temperature in the second reactor, which lessens the effectiveness of the vinyl modifier.
There remains a need for a process to produce polymers having high vinyl segments on all ends, which does not require the use of expensive and unstable organo-dilithium initiators. Furthermore, there remains a need for a process in which a higher vinyl content may be obtained on the end segments without increasing the vinyl content of the middle segment. Additionally, there remains a need for a process that is capable of providing a more abrupt change in vinyl content without the need for a temperature differential to lessen the effectiveness of the vinyl modifier.